The present invention relates to a method and apparatus for continuously controlling the flow-rate through a well.
More particularly, the present invention relates to a continuously circulation drilling method, which may be also carried out as the drilling rods are replaced or exchanged, by means of a special valve battery and an additional mud switching circuit for switching muds, while preventing any water hammering events.
As is known, a drilling operation is affected by the following main operating problems:
reservoir kick phenomena conveying to a ground surface highly pressurized reservoir gases or fluids;
well absorption phenomena during the drilling operations, causing a reservoir perforating mud dispersion and related economic/environmental problems;
a complex control of the mud material introduced into the well;
a complex control of the outlet mud characteristics;
a very dangerous re-rising of gases;
the requirement of performing in a safe condition the rod exchanging operations; and
a complex control of all physical and fluidynamic parameters of the well.
In prior mud-circulating and drilling methods, have already been used different control systems for controlling the well inlet and outlet flow rates, to detect the well kick or absorbing phenomena. The well inlet drilling mud flow rate Is set by a system pump to be a constant one, whereas the standard well outlet flow rate, in the absence of well failures, should be equal to the inlet flow rate (with a set measurement error tolerance).
Prior studies on the well reservoir drilling performance have shown that, in many cases, the well outlet flow rate was not constant and, moreover, not comparable with the well inlet flow rate.
Such a variation is due to phenomena occurring in the well and which in some cases can negatively affect the drilling result.
Moreover, prior well control systems for controlling mud circulating drilling methods monitor the well inlet and outlet flow-rates and operating pressures, by means of the choke valves, properly monitoring the well backpressure and dangerous related events.
However, said prior systems do not comprise fluid control means for controlling the switching off of the system pumps for exchanging the drilling string rods.
Since the above mentioned “kick” phenomenon derives from the fact that it is not possible to hold in the well bore a constant pressure, any increase and decrease of the pressure on the bore walls causes a hydraulic fracturing, even at undesired positions.
Moreover, a continuous circulation prevents debris or waste material from falling down bore, thereby the waste materials continuously moves upward the well, in turn preventing any well battery engaging, with consequent time equipment or well losses.
Prior measurement systems for carrying out mud flow rate measurements, just limited by the mud characteristics, comprise mass measuring Coriolis devices, (designed for measuring the mud flow rate and density), or Venturi measuring devices, operating based on a Venturi momentum variation.
To the above it should be also added that some oil Companies commercially use a flow rate and density measuring control system based on a Coriolis' Meter, the drilling system being a software so controlled as to prevent kick phenomena.
Said prior systems are affected by great limitations since a Coriolis' measuring device, in presence of gas, is not able of measuring and detecting the gas presence and, accordingly, it is necessary to manually drive the systems, thereby the measured data must be further evaluated by an operator (for detecting if a well kick or absorbing phenomenon is being occurring).
Moreover, said prior systems do not detect the well inlet flow rate, which is instead calculated by using the pump operating strokes which calculation, on the other hand, (even if from an operator standpoint could constitute a set point of reference) could be dangerous and misleading.
In fact, for properly measuring well flow rates by pump strokes, it is necessary to know the pump efficiency, as well as to predict possible pump failures.
Thus, a pump stroke method is considered a redundant measurement method for detecting the pump efficiency and operation capability, but it cannot be used as a reference measuring method.
Moreover, prior measurement Coriolis systems are very expensive and must be subjected to periodic servicing operations to prevent measurement vibrating pipes from being clogged.